1. Field of the Invention
The present invention relates to an optical receiver for optical communication and a method of producing the optical receiver, and in particular to an optical receiver having high sensitivity, a capability to perform high-speed response, and superior productivity and a method of producing the optical receiver.
2. Description of the Background Art
FIGS. 7(A) and 7(B) are vertical cross sections showing the structure of a conventional optical receiver. FIG. 7(A) is a sectional front view, and FIG. 7(B) is a sectional side view at the B—B section shown in FIG. 7(A). In FIGS. 7(A) and 7(B), an optical receiver 100 is known as a pigtail type using a coaxial type package. The optical receiver 100 is assembled by the following process. A submount 102 is fixed onto a package 103. A photodiode (PD) 101 used as a light-receiving device is fixed onto the submount 102 by soldering. A plurality of lead pins 104 penetrate the package 103 to feed electric power and to take out electric signals. Wires 105 connect between the lead pins 104 and the PD 101 and between the lead pins 104 and the submount 102. A cap 107 provided with a condenser lens 106 at the top covers the submount 102. An optical fiber 108 is securely held at a place over the lens 106. A cover is applied over the package 103 to complete the assembly. The cover is shown in FIGS. 7(A) and 7(B) by a broken line that connects the optical fiber 108 and the package 103. In the optical receiver 100, the PD 101 receives incoming light emerging from the optical fiber 108 after it passes through the condenser lens 106.
In the foregoing optical receiver using a coaxial type package, a front-illuminated type light-receiving device is usually used. The front-illuminated type light-receiving device is formed by the following manner, for example. FIG. 8 is a cross-sectional view schematically showing a front-illuminated type light-receiving device. A light-receiving layer 111 made of n-type InGaAs is epitaxially grown on an n-type InP substrate 110. A p-type Zn-diffused layer 112 is formed at the central portion of the light-receiving layer 111 to form a p-n junction. An SiNx layer 113 is formed on the light-receiving layer 111 to protect the p-n junction. An n-type electrode 114 is formed on the back face of the substrate 110. A p-type electrode 115 is formed on the Zn-diffused layer 112. This concludes the formation of the front-illuminated type light-receiving device. Additionally, a bonding pad 116 to be used as a space for bonding a wire 105 is formed such that the bonding pad 116 covers a part of the SiNx layer 113 with maintaining an electrical connection with the p-type electrode 115. Such a front-illuminated type light-receiving device usually introduces light from the front side where the Zn-diffused layer 112 is exposed as shown in FIG. 8.
On the other hand, an optical module using a rear-illuminated type light-receiving device is also known as shown by published Japanese patent application Tokukaihei 7-199006. FIG. 9 is a cross-sectional view schematically showing a rear-illuminated type light-receiving device. The rear-illuminated type light-receiving device is formed by the following process, for example. As with the front-illuminated type light-receiving device, a light-receiving layer 121 made of n-type InGaAs is epitaxially grown on an n-type InP substrate 120. A p-type Zn-diffused layer 122 is formed at the central portion of the light-receiving layer 121 to form a p-n junction. An SiNx layer 123 is formed on the light-receiving layer 121 to protect the p-n junction. An n-type electrode 124 is formed on the back face of the substrate 120 such that the n-type electrode 124 has a proper opening to expose a part of the substrate 120 so that incoming light can be introduced. A p-type electrode 125 is formed to cover the central portion of the Zn-diffused layer 122. This concludes the formation of the rear-illuminated type light-receiving device. As described above, a rear-illuminated type light-receiving device usually introduces light from the rear side where the substrate 120 is exposed as shown in FIG. 9. In this device, the top surface of the p-type electrode 125 is plated with gold or another material (not shown in FIG. 9) to allow the direct bonding of a wire 105 as shown in FIG. 9.
The optical communications system has been widely used in recent years. This situation requires that the optical receiver be mass-produced at a lower cost in a shorter time. The transmission capacity of the communications system is also required to increase from the conventional 156 Mbps and 622 Mbps or so to 1 Gbps and 2.5 Gbps or even higher to achieve high-speed, large-capacity transmission.
As described above, an optical receiver incorporating a coaxial type package usually uses a front-illuminated type light-receiving device, which is easy to mount in such a way that the light-receiving layer faces the incoming light in a direction perpendicular to the optical axis of the incoming light. In spite of this advantage, the front-illuminated type light-receiving device has a response speed no more than 1 Gbps or so. In other words, the device has an undesirable limitation to be used for a higher-speed and larger-capacity transmission.
On the other hand, the rear-illuminated type light-receiving device is known as a suitable device for high-speed response. However, when this type of device is used in the coaxial type package, it is necessary to provide an optical path-forming groove or another means in the submount in order to introduce incoming light. Generally, the formation of the groove and other means reduces the productivity. The above-mentioned Tokukaihei 7-199006 has no description on the optical receiver incorporating the coaxial type package.